I was in Pottery Barn this weekend and they had them...on sale.

I am not familiar with the stores in the UK. However, if you are unable to find them locally, consider having them custom made. Being a seamstress myself, that is naturally my solution.

If you have a sewing machine or know someone who does, this is a super easy job. Just take off the existing canvas and copy the sizes, hem the edges and you are done. Just be sure to use synthetic thread and outdoor fabric if you are using these chairs outdoors.

I found one place, but it looks like it would be almost cheaper to buy a new chair. Making your own would be much cheaper.